In recent years, research of optical materials is intensively made, and particularly, in the field of lens materials, development of materials excellent in a high refractive index, low dispersibility (i.e. high Abbe number), heat resistance, transparency, easy moldability, lightness, moisture resistance, chemical resistance, solvent resistance, and the like is strongly demanded. Plastics (resins) are light and hardly cracked and can be easily processed into a variety of shapes, as compared with inorganic materials such as glasses. From a view point of easy processing, the plastics have been rapidly spread in not only spectacle lenses and lenses for cameras, but also in recent years, particular-shaped optical materials such as display panel utility. On the other hand, since the resins have generally a low refractive index as compared with glasses, a higher refractive index of materials themselves is demanded in order to thin optical members.
Therefore, in the field of display panels, provision of a metal oxide particle-complexed resin which has a high refractive index and is excellent in transparency by containing particles of a metal oxide such as titanium oxide and zirconium oxide in the resin has been proposed. For example, since zirconium oxide has a high refractive index peculiar to a metal oxide, by complexing with the resin, application to high refractive index optical materials is expected.
In order to manufacture a metal oxide particle-complexed resin which has a high refractive index and is excellent in transparency, it is necessary to sufficiently reduce a size of metal oxide particles in a complexed resin as compared with a wavelength of visible light, in order to prevent light scattering due to the metal oxide particles in the complexed resin. Specifically, by complexing nanoparticles of a metal oxide having a particle diameter of 100 nm or less with a resin while retaining the particle diameter, it is expected that a refractive index of optical materials can be improved while maintaining transparency.
On the other hand, it has been known that nanoparticles of a metal oxide have hydrophilicity. Therefore, in order to cause such hydrophilic nanoparticles to be uniformly contained in a hydrophobic solvent or a hydrophobic resin without aggregating the nanoparticles, treatment of hydrophobizing a surface thereof to enhance affinity with an organic solvent or a resin becomes necessary.
Hydrophobization of a surface of nanoparticles of a metal oxide is performed by a variety of methods.
For example, Japanese Unexamined Patent Application, First Publication No. 2010-159464 (Patent Document 1) describes nanoparticles which were hydrophobized by covering a surface with a phosphoric acid-based surfactant having a specified structure.
Also, Japanese Unexamined Patent Application, First Publication No. 2008-201634 (Patent Document 2) describes a zirconium oxide particle-containing material (dispersion) having light transmittance in terms of the thickness of 10 mm at a wavelength of 589 nm of 80% or more, which contains zirconium oxide particles having a number average particle diameter of 1 to 15 nm, a dispersant and a dispersing medium (organic solvent). It is stated that in this containing material, a refractive index of a composition of the zirconium oxide particles and the dispersant other than the dispersing medium is 1.80 or more.
Furthermore, Japanese Unexamined Patent Application, First Publication No. 2009-024068 (Patent Document 3) describes a process for manufacturing resin particles comprising nanoparticles of zirconium oxide obtained by containing (dispersing) liquid droplets comprising nanoparticles of covered zirconium oxide covered with two or more covering agents (comprising at least a carboxylic acid of a hydrocarbon having 6 or more carbon atoms), and a monomer in an aqueous solvent, and conducting a polymerization reaction in the liquid droplets.
Japanese Unexamined Patent Application, First Publication No. H08-110401 (Patent Document 4) describes a transparent high refractive index film consisting of an organic polymer comprising metal compound microparticles having a refractive index of 1.70 to 2.70.